Thank you so for taking the time to reply. It turned out his symptoms disappeared shortly after my post. It was a great lesson and a wake-up call, as well as your message.
In order to make sure if you or your partner actually have an STD, you will need to be tested for it.
Also keep in mind that just because you may be asymptomatic, doesn't mean that it's Trichomoniasis... it could just as easily be Chlamydia, Gonorrhea, UreaPlasma or Mycoplasma - they all have relatively similar symptoms across the board that it can be difficult to narrow it down by symptoms alone.
On the other hand... it doesn't have to be STD related, but rather mild irritation from fecal bacteria as you said... or something else.
You will have to get tested to make sure as will your partner... and in addition, I recommend you both do a Syphilis test in addition to the above mentioned tests.
P.S. How do you know your partner is monogamous?
You aren't, and unless he found out without telling you, he doesn't know (plus if he thinks along similar lines like you do, its possible he might assume the same of you - that you are also monogamous).
Making assumptions can be rather problematic because people often leap to conclusions which have no basis in reality.
Instead of actively questioning things, people conjure up a personal theory based on their own preconceived notions/bias and project it outwards - this is extremely shortsighted and results in more problems than it solves.
Its one thing to conjure up a theoretical explanation about a subject matter, but to actually think that this is how things occurred without testing it in practice is (again) shortsighted.
Also... just because you were not monogamous doesn't mean that you might have given him something - though given the available data (and without knowing if your partner is also playing on the side), it 'might' have been you - but that only applies if he's actually infected with something (he doesn't have to be).
As I said... both of you will have to do full blown testing to ascertain whether you have bacterial STD's or not.
For syphilis, you would have to wait about 6 weeks after your last sexual encounter... but for others, about 7 days of waiting will be enough.
But as you said yourself, this doesn't have to be STD related.